A wide variety of implantable medical devices (IMDs) that employ electronic circuitry for providing electrical stimulation of body tissue and/or monitoring a physiologic condition are known in the art. A number of IMDs of various types are known in the art for delivering electrical stimulating pulses to selected body tissue and typically comprise an implantable pulse generator (IPG) for generating the stimulating pulses under prescribed conditions and at least one lead bearing a stimulation electrode for delivering the stimulating pulses to the selected tissue. For example, cardiac pacemakers and ICDs have been developed for maintaining a desired heart rate during episodes of bradycardia or for applying cardioversion or defibrillation therapies to the heart upon detection of serious arrhythmias. Other nerve, brain, muscle and organ tissue stimulating medical devices are also known for treating a variety of conditions. The present invention will be described in relation to ICDs, but it is not intended that the invention be limited to that particular application when it can be advantageously implemented in other implantable medical devices.
In their simplest forms, cardiac pacemaker and ICD IPGs typically are formed having a metallic housing that is hermetically sealed and, therefore, is impervious to body fluids, and a header or connector assembly for making electrical and mechanical connection with one or more leads bearing pacing, sensing and cardioversion/defibrillation electrodes adapted to be located in or around selected chambers of the heart. Over the past 20 years, ICD IPGs have evolved, as described in some detail in commonly assigned U.S. Pat. No. 5,265,588, incorporated herein by reference in its entirety, from relatively bulky, crude, and short-lived IPGs simply providing high energy defibrillation shocks to complex, long-lived, and miniaturized IPGs providing a wide variety of pacing, cardioversion and defibrillation therapies. Numerous other programmable functions have been incorporated including enhanced capacity to detect and discriminate cardiac arrhythmias, data storage and uplink telemetry of data related to arrhythmia episodes and applied therapies, provision of staged therapies appropriate to the detected arrhythmia, for example. At the same time, numerous improvements have been made in cardioversion/defibrillation leads and electrodes that have enabled the cardioversion/defibrillation energy to be precisely delivered about selected upper and lower heart chambers and thereby dramatically reducing the delivered shock energy required to cardiovert or defibrillate the heart chamber. Moreover, the high voltage output circuitry has been improved in many respects to provide monophasic, biphasic, or multi-phase cardioversion/defibrillation shock or pulse waveforms that are efficacious, sometimes with particular combinations of cardioversion/defibrillation electrodes, in lowering the required shock energy to cardiovert or defibrillate the heart.
Throughout the course of development of these improvements, successive generations of such IPGs have always included common components located within the ICD IPG housing. These components include one or more battery, one or more high power cardioversion/defibrillation output capacitor, low voltage electronic circuitry powered by a battery for detecting and discriminating pathologic and/or nonpathologic arrhythmias from one another and providing other functions, and high voltage electronic charging circuitry for charging the output capacitor(s) from a battery voltage to a higher voltage and electronic switching circuitry for dumping the charge built up on the output capacitor(s) through the cardioversion/defibrillation electrodes. The charging circuitry typically comprises a DC-DC, "flyback" converter employing a bulky step-up transformer and switching circuitry of the type disclosed in the above-incorporated '588 patent that converts low battery voltage to a programmed high voltage that the output capacitor(s) are charged to.
It is widely understood that such ICD IPGs need to be small enough to be comfortably implanted subcutaneously without being unduly uncomfortable to the patient or cosmetically apparent. The first implanted automatic implantable defibrillator (AID) IPG housing disclosed in U.S. Pat. No. 4,254,775 was very large and had to be implanted in a patient's abdominal region. The two cylindrical output capacitors, the redundant, rectangular, high voltage batteries and the circuit boards bearing discrete components and integrated circuits (ICs) depicted in the '775 patent drawings are very bulky and assembled together by a framework that left a great deal of unused or unfilled space within the housing. These spaces exist because these components were only available in these shapes, whereas it is necessary to provide the IPG housing with rounded sides that provide more gradual transitions to prevent the housing edges from causing tissue erosion at the implant site.
Since that time, the ICs have been vastly reduced in size while their complexity has been vastly increased. Battery energy requirements for powering both the low voltage ICs and for providing the cardioversion/defibrillation shocks have been reduced while battery energy density has been increased and battery configuration made more conforming to the interior space of the IPG housing. Miniaturized, flat high voltage output capacitors that can be shaped to fit the allocated housing space and miniaturized high voltage switching components have been developed and employed. All of these improvements, together with the above-mentioned cardioversion/defibrillation improvements have contributed to a significant reduction in the volume of the IPG housing. Some of these improvements in capacitors and batteries are described in U.S. Pat. Nos. 5,370,663, 5,370,669, 5,405,363, 5,527,346, 5,749,911, and 5,827,326, incorporated herein by reference. A space conserving, energy dissipation resistor is disclosed in U.S. Pat. No. 5,312,442, incorporated herein by reference in its entirety.
As these components are miniaturized to fit tightly within the confines of the ICD housing, the size and shape of the step-up transformer becomes a limiting factor on the thickness of the generally flat hermetically sealed ICD housing as shown in the above-incorporated '669 patent (transformer 76). Its bulk and shape can also lead to wasted space around it within the housing as shown in the above-incorporated '326 patent (FIG. 4).
It is a principal object of the present invention to provide further improvements in miniaturization of components employed within an IMD housing to fill one or more space within the housing interior that would otherwise not be occupied.